1. Field of the Invention
This invention relates generally to a Time Division Multiplexed (TDM) digital communications satellite system and more particularly to a technique for transmitting bit-stuffing decisions in an asynchronous TDM satellite system.
2. Description of the Prior Art
In a TDM digital satellite communications system having a plurality of earth stations each earth station may be required to accept data inputs from several terrestrial sources whose information rate varies relative to the TDM system information rate. Though the difference in information rate may vary the system design insures that the TDM system information rate is higher than the data information rate. To accommodate the difference in information rate the TDM system inserts extra pulses (pulse stuffing) in the incoming data in order to equalize the bit rates. At a receiver the stuffed pulses, which carry no information, are removed when the signal is demultiplexed.
The decision to stuff or not to stuff must be conveyed to the receiver. If the decision to stuff is made, then the receiver must be so instructed in order that the stuffed pulses may be properly removed. The stuff decision is binary (yes or no) and can be conveyed by only one bit. However, there is a finite probability that channel noise would corrupt this bit so that the stuff decision bit will be received in error. When an error occurs in the transmission of stuff decision, blocks of data will be processed incorrectly thereby degrading the communication channel performance significantly.
To reduce the probability of error in transmitting the stuff decision below the probability of channel noise corrupting a bit, i.e., below the probability of channel bit error, a standard approach is to encode the stuff decision into a digital word of multiple bit length. The receiver will detect this multiple bit word if the number of bit errors is below a threshold determined by the desired probability of detecting the stuff decision correctly. However, in a TDM digital satellite communications system wherein each earth station transmits, in one time frame, a burst of information, each bit that is allocated to the stuff decision word (signalling) is a bit in the burst time slot that is no longer available to carry information. If the efficiency of the system is defined as the ratio of available information bit time slots in a burst to the length of the burst then every bit added to the stuff decision word will decrease the efficiency of the system.
The present invention has the advantage of transmitting stuff decision information through a TDM digital satellite communications system with a probability of error much less than the bit error probability of the communication channel while requiring a minimum of one information bit per burst for each earth station.